Bind MailgunSender to ISender in singleton scope.

[brnn8r]

Bind MailgunSender to ISender in singleton scope.

MailgunSender creates a new HttpClient for each instance of a MailgunSender object.

Based on Microsoft recommendations https://learn.microsoft.com/en-us/dotnet/fundamentals/networking/http/httpclient-guidelines#recommended-use I propose the MailgunSender should be bound in Singleton scope so that only one HttpClient is instantiated.